Purpose The aim of this study was to compare the macular thickness and each retinal layer thickness of the amblyopic eyes with their normal fellow eyes in patients with unilateral amblyopia using the Spectralis spectral domain optical coherence tomography new segmentation software.Patients and methods A total of 20 patients with unilateral amblyopia were enrolled in this study. Seventeen patients (85%) had anisometropic amblyopia, and three (15%) had combined amblyopia (strabismic and anisometropic). Best-corrected visual acuity was measured and converted to the logarithm of the minimum angle of resolution (logMAR). Patients underwent a comprehensive ophthalmic examination, including Spectralis spectral domain optical coherence tomography scanning. The mean of each of the three macular thickness map circles and the mean thickness of each of the retinal layers at the fovea and at 1000 μm circle and the 2500 μm circle were automatically extracted using Spectralis new segmentation software.Results There was a statistically significant increase in the macular thickness at the central subfield region of the early treatment diabetic retinopathy study (ETDRS) map in the amblyopic eyes (288.65±22.61 μm) compared with the fellow normal eyes (281.1±22.6 μm). In addition, there was a statistically significant increase in the outer nuclear layer thickness at the fovea of the amblyopic eyes (114.7±13.93 μm) compared with the fellow normal eyes (104.4±15.63 μm). There was also a statistically significant increase in the thickness of the retinal nerve fiber layer at the outer circle of the amblyopic eyes (33.03±3.816 μm) compared with the fellow normal eyes (30.76±3.75 μm). However, there was a statistically significant decrease in the thickness of the ganglion cell layer at the outer circle in the amblyopic eyes (36.38±5.27 μm) compared with the fellow normal eyes (39.09±6.19 μm).Conclusion The increase in the thickness of the central subfield region in the macular thickness map and the outer nuclear layer at the fovea implies that the photoreceptors may be affected by amblyopia.

Amblyopia remains an important cause of low visual acuity, affecting 2–6% of the general population. Classic causes include strabismus, anisometropia, and form deprivation or a combination of these factors [1].

The site of pathology in amblyopia has been controversial: whether the origin is retinal or cortical. Despite some early studies indicating that the retina might be the primary site of pathology in amblyopia, the conclusions reached from a wide range of animal studies and electrophysiological investigations in humans were that the retina demonstrates essentially normal physiologic functions in the presence of amblyopia [2],[3]. However, many questions remain: whether the retina of amblyopic patients is normal or the abnormality in the visual cortex is the primary and only cause of amblyopia [4],[5]. Recent advances in neuroanatomy, neurophysiology, and optical imaging have reopened the possibility that there is some retinal dysfunction in amblyopia [5],[6].

The new technique for automated retinal segmentation using the Spectralis spectral domain optical coherence tomography (SD-OCT) device can now be used to automatically identify each retinal layer and quantify its thicknesses in each single horizontal retinal scan (segmentation technology; Heidelberg Engineering, Heidelberg, Germany). Thus, the retina can be segmented into 10 layers, permitting a more detailed assessment of pathologic changes in the retina [7].

Recently, several examinations of the amblyopic retinal thickness using SD-OCT have been reported. In some of these reports, there were no differences in the retinal thickness between amblyopic and fellow eyes [8],[9], whereas, in others, the amblyopic eyes were found to be either thicker or thinner compared with the fellow eyes [10],[11],[12]. Therefore, a consensus has not yet been obtained based on OCT, and the degree of retinal involvement accompanying amblyopia is controversial.

The aim of this study was to compare the macular thickness and each retinal layer thickness of the amblyopic eyes with their normal fellow eyes in patients with unilateral amblyopia using the Spectralis SD-OCT new segmentation software.

Patients and methods

This prospective study was performed at the Department of Ophthalmology, at Alexandria Main University Hospital, on 20 Egyptian patients (11 male and nine female; age range 15–40 years) with unilateral amblyopia who presented to the outpatient clinic for examination. Exclusion criteria were as follows: presence of diabetes mellitus, systemic inflammatory disease, ocular abnormalities (media opacity, retinal pathology, glaucoma, or uveitis), recent ocular trauma, and congenital anomalies. The study was approved by the Institutional Board Review of the Faculty of Medicine, Alexandria University. An informed consent was signed by all participants before being enrolled in the study.

The Spectralis SD-OCT (Heidelberg Engineering) was used to measure the macular thickness map, which consists of three concentric rings with diameters 1 (central subfield), 3 (inner circle), and 6 (outer circle) mm and their mean thickness was calculated ([Figure 1]). The new prototype for retinal segmentation using the Heidelberg Eye Explorer software (version 6.0.0.2) was used to extract the thickness of eight retinal layers: the retinal nerve fiber layer (RNFL), the ganglion cell layer (GCL), the inner plexiform layer (IPL), the inner nuclear layer (INL), the outer plexiform layer (OPL), the outer nuclear layer (ONL), photoreceptors, and retinal pigment epithelium (RPE) ([Figure 2]). The mean thickness of each retinal layer was analyzed at the fovea and at the 1000 μm circle and the 2500 μm circle in each of the four quadrants (superior, inferior, nasal, and temporal).

The thickness of each layer was compared between normal and amblyopic eyes. The data were analyzed using the Statistical Package for Social Sciences (version 20; SPSS Inc., Chicago, Illinois, USA). The distributions of quantitative variables were tested for normality using the Kolmogorov–Smirnov test, which revealed that the data were normally distributed. Quantitative data were described using mean and SD. Quantitative variables between the two eyes were compared using paired t-test. In all statistical tests, level of significance of 0.05 was used, below which the results were considered statistically significant.

Results

This study included 20 patients with unilateral amblyopia. Seventeen patients (85%) had anisometropic amblyopia, and three (15%) had combined amblyopia (strabismic and anisometropic). Fifteen patients (75%) were between 15 and 30 years of age and five (25%) patients were between 30 and 40 years of age, with a mean of 25.3±8.48 years. The degree of anisometropia was between 2 and 6 D in most of the patients (80%). The range of BCVA in the amblyopic eyes was 20/40–20/200. The mean BCVA was +0.69±0.39 logMAR in the amblyopic eyes and 0.00 in the fellow eyes, which had a BCVA of 20/20.

There was a statistically significant increase in the macular thickness at the central subfield region in the ETDRS map of the amblyopic eyes (288.65±22.61 μm) compared with the fellow normal eyes (281.1±22.6 μm) ([Table 1]).

There was a statistically significant decrease in the thickness of the GCL at the outer circle in the amblyopic eyes (36.38±5.27 μm) as compared with the fellow normal eyes (39.09±6.19 μm) ([Table 3]).

The other retinal layers including the IPL, the INL, the OPL, photoreceptors, and RPE showed no statistically significant difference between amblyopic and normal fellow eyes.

Discussion

In the present study, as regards the macular thickness, the mean central subfield was found to be thicker in the amblyopic eyes (288.65±22.61 μm) than in the fellow normal eyes (281.1±22.6 μm). Similar results were reported by other studies. In 2011, Alotaibi and Al Enazi evaluated 93 unilateral amblyopic eyes (36 strabismic, 33 anisometropic, and 24 combined). They found a higher macular and foveal thickness only in the anisometropic amblyopic group (macular thickness: 256.76 vs. 246.61 μm, P=0.050; foveal thickness: 187.12 vs. 177.61 μm, P=0.039), whereas there was no significant difference in macular and foveal thickness in the strabismic amblyopic group [13]. Similarly, Al-Haddad et al. used one single horizontal SD-OCT scan for the manual segmentation of six layers of the central 1000 μm diameter area, and found that the mean foveal thickness was increased in amblyopic eyes compared with the normal fellow eyes (228.5±20.2 vs 221.7±15.3 μm) [11].

Other studies, however, reported no significant difference in the mean macular thickness. Kee et al.[8] enrolled 26 unilateral amblyopic children (six strabismic, 15 anisometropic, and five combined amblyopes) and found no difference in foveal thickness between neither the amblyopic eye and fellow eye nor between values of these amblyopic patients and 42 normal control children, using time-domain OCT. In 2005, Altintas et al.[9] carried out OCT examination on 14 unilateral strabismic amblyopic patients and found no difference in macular thickness.

In the current study, RNFL layer thickness was not statistically significantly different between amblyopic and normal fellow eyes at the inner circle. However, at the outer circle, there was an increase in the mean thickness of the RNFL of the amblyopic eyes (33.03±3.816 μm) compared with the normal fellow eyes (30.76±3.75 μm).

Some studies reported similar results. In the study by Alotaibi and Al Enazi [13], they found significantly thicker RNFL (259.3 vs. 255.6 μm, P<0.0001) in the overall amblyopic group. Similarly, Yen and colleagues, in 2004, used Cirrus OCT machine to measure RNFL thickness in 38 patients (mean age 26.4; range 6–75 years) with unilateral amblyopia (strabismic and refractive amblyopia) and found no significant difference between strabismic amblyopic and normal eyes. However, the RNFL was significantly thicker in eyes with refractive amblyopia compared with the fellow eyes [14].

However, some studies reported that there was no statistically significant difference in the thickness of the RNFL between the amblyopic eyes and the fellow normal eyes. In the study by Kee et al.[8], there was no difference in RNFL thickness in any of the examined four quadrants (superior, inferior, nasal, and temporal), nor between values of these amblyopic patients and 42 normal control children, using time-domain OCT. In addition, in the study by Altintas et al.[9], there was no difference in RNFL thickness between amblyopic and sound eyes.

In the present study, there was a statistically significant decrease in the thickness of the GCL of the amblyopic eyes (36.38±5.27 μm) compared with the normal fellow eyes (39.09±6.19 μm) only at the outer macular circle.

Some studies showed similar results. Park and colleagues enrolled 20 unilateral amblyopic children (16 strabismus, two anisoastigmatism, and two unilateral ptosis) with a mean age of 9.0±4.03 years (range 4–19 years) and examined horizontal and vertical spectral-domain OCT scans through the fovea. Thickness values were measured at the foveal center and in 500 and 1500 μm distance from the foveal center in all four quadrants (superior, inferior, nasal, and temporal). The thickness of each retinal layer (GCL+IPL, INL, OPL, ONL, IS, OS, and RPE) was measured manually using the calipers provided with the SD-OCT instrument. They found significantly decreased thickness in the thickness of the GCL+IPL at all four nasal and temporal macular locations and at the outer superior and inferior locations [11].

In contrast, Tugcu et al.[15] used the built-in analysis option of the RTVue OCT platform to measure the thickness of the GCL and found an increase in strabismic amblyopic eyes, whereas there was no such difference for the anisometropic or combined subgroups.

In the present study, the ONL was found to be significantly thicker in the amblyopic eyes (114.7±13.93 μm) than in the fellow normal eyes (104.4±15.63 μm) at the fovea.In accordance with the present result, Szigeti and colleagues enrolled 38 consecutive patients with unilateral amblyopia in a study (16 male; mean age 32.4±17.6 years; range 6–67 years) in 2014. OCT examinations were performed with a time-domain OCT device, and a custom-built OCT image analysis software (OCTRIMA) was used for OCT image segmentation. They found a significant difference in ONL thickness in the central region (P=0.032). After implementing an ideal axial length of 22.73 mm and an age of 32.79 years, the estimated ONL thickness would be 120.69 μm in the amblyopic eye, which is greater than 120.28 μm in the fellow eye [12].

On the other hand, Park et al.[11] found that the ONL was thinner at the inner and outer temporal locations and thicker at the inner and outer superior and inner nasal locations. Al-Haddad et al.[10] using one single horizontal SD-OCT scan for the manual segmentation of six layers of the central 1000 μm diameter area found a decrease in the ONL in the temporal area in amblyopic eyes compared with the fellow eyes.

The increase in ONL thickness at the fovea, in the present study, implies that the photoreceptors and not the ganglion cells could be affected by amblyopia. In support of the present results, there is early evidence showing that photoreceptors may be affected in amblyopia. Enoch [16] was the first of many authors to suggest a specific cause for an organic anomaly affecting the retina in amblyopia, and suggested that photoreceptor orientation is abnormal in amblyopic eyes using the Stiles-Crawford function.

Conclusion

This study measured the macular thickness in unilateral amblyopia using Spectralis SD-OCT image segmentation software of the entire macular area. Using this methodology an increase in the macular thickness at the central subfield region and the ONL at the fovea in the amblyopic eyes compared with the fellow normal eyes was detected suggesting the possible involvement of the photoreceptors.